Re: [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to race with CPU offline

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 27, 2018 at 11:11:06AM +0200, Peter Zijlstra wrote:
> On Tue, Jun 26, 2018 at 04:40:04PM -0700, Paul E. McKenney wrote:
> > The options I have considered are as follows:
> 
> > 2.	Stick with the no-failsafe approach, but rely on RCU's grace-period
> > 	kthread to wake up later due to its timed wait during the
> > 	force-quiescent-state process.  This would be a bit obnoxious,
> > 	as it requires passing a don't-wake flag (or some such) up the
> > 	quiescent-state reporting mechanism.  It would also needlessly
> > 	delay grace-period ends, especially on large systems (RCU scales
> > 	up the FQS delay on larger systems to maintain limited CPU
> > 	consumption per unit time).
> > 
> > 3.	Stick with the no-failsafe approach, but have the quiescent-state
> > 	reporting code hand back a value indicating that a wakeup is needed.
> > 	Also a bit obnoxious, as this value would need to be threaded up
> > 	the reporting code's return path.  Simple in theory, but a bit
> > 	of an ugly change, especially for the many places in the code that
> > 	currently expect quiescent-state reporting to be an unconditional
> > 	fire-and-forget operation.
> 
> Here's a variant on 2+3, instead of propagating the state back, we
> completely ignore if we needed a wakeup or not, and then unconditionally
> wake the GP kthread on the managing CPU's rcutree_migrate_callbacks()
> invocation.
> 
> Hotplug is rare (or should damn well be), doing a spurious wake of the
> GP thread shouldn't matter here.

Another variant, which simply skips the wakeup whever ran on an offline
CPU, relying on the wakeup from rcutree_migrate_callbacks() right after
the CPU really is dead.

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 7832dd556490..417496a03259 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -104,7 +104,6 @@ struct rcu_state sname##_state = { \
 	.abbr = sabbr, \
 	.exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
 	.exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
-	.ofl_lock = __SPIN_LOCK_UNLOCKED(sname##_state.ofl_lock), \
 }
 
 RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
@@ -1928,13 +1927,11 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 	 */
 	rsp->gp_state = RCU_GP_ONOFF;
 	rcu_for_each_leaf_node(rsp, rnp) {
-		spin_lock(&rsp->ofl_lock);
 		raw_spin_lock_irq_rcu_node(rnp);
 		if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
 		    !rnp->wait_blkd_tasks) {
 			/* Nothing to do on this leaf rcu_node structure. */
 			raw_spin_unlock_irq_rcu_node(rnp);
-			spin_unlock(&rsp->ofl_lock);
 			continue;
 		}
 
@@ -1970,7 +1967,6 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 		}
 
 		raw_spin_unlock_irq_rcu_node(rnp);
-		spin_unlock(&rsp->ofl_lock);
 	}
 	rcu_gp_slow(rsp, gp_preinit_delay); /* Races with CPU hotplug. */
 
@@ -2250,11 +2246,19 @@ static int __noreturn rcu_gp_kthread(void *arg)
 static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
 	__releases(rcu_get_root(rsp)->lock)
 {
+	int cpu = smp_processor_id();
+
 	raw_lockdep_assert_held_rcu_node(rcu_get_root(rsp));
 	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
 	WRITE_ONCE(rsp->gp_flags, READ_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS);
 	raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags);
-	rcu_gp_kthread_wake(rsp);
+
+	/*
+	 * When our @cpu is offline, we'll get a wakeup from
+	 * rcutree_migrate_callbacks.
+	 */
+	if (cpu_online(cpu))
+		rcu_gp_kthread_wake(rsp);
 }
 
 /*
@@ -3768,18 +3772,15 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp)
 
 	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */
 	mask = rdp->grpmask;
-	spin_lock(&rsp->ofl_lock);
 	raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
 	rdp->rcu_ofl_gp_seq = READ_ONCE(rsp->gp_seq);
 	rdp->rcu_ofl_gp_flags = READ_ONCE(rsp->gp_flags);
+	rnp->qsmaskinitnext &= ~mask;
 	if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
-		/* Report quiescent state -before- changing ->qsmaskinitnext! */
 		rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	}
-	rnp->qsmaskinitnext &= ~mask;
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
-	spin_unlock(&rsp->ofl_lock);
 }
 
 /*
@@ -3849,6 +3850,12 @@ void rcutree_migrate_callbacks(int cpu)
 {
 	struct rcu_state *rsp;
 
+	/*
+	 * Just in case the outgoing CPU needed to wake the GP kthread
+	 * do so here.
+	 */
+	rcu_gp_kthread_wake(rsp);
+
 	for_each_rcu_flavor(rsp)
 		rcu_migrate_callbacks(cpu, rsp);
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 4e74df768c57..8dab71838141 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -367,10 +367,6 @@ struct rcu_state {
 	const char *name;			/* Name of structure. */
 	char abbr;				/* Abbreviated name. */
 	struct list_head flavors;		/* List of RCU flavors. */
-
-	spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
-						/* Synchronize offline with */
-						/*  GP pre-initialization. */
 };
 
 /* Values for rcu_state structure's gp_flags field. */